1. Field of the Invention
The present invention relates to lightweight rigid structural panels such as are used for interior walls, floors, and doors in aircraft, and more particularly to such panels as are designed to resist ballistic impacts from bullets, shrapnel, and like objects.
2. Description of Related Art
Lightweight rigid structural panels have long been used to construct walls, floors, and other structures in the interior of aircraft. These panels are often made by laminating sheets of structural materials such as aluminum or fiber/resin composites around a lightweight core, such as a honeycomb or foam material. The laminated structural panels possess high stiffness and favorable strength-to-weight ratios, making them well-suited for handling ordinary interior loads and uses in aircraft. Prior art interior panels suitable for aircraft have not been available for resisting destruction and penetration by ballistic objects such as bullets and shrapnel, or for resisting deformation from high force/low speed attack by objects such as battering rams, knives, cutting tools, and pry-bars. In the past, such capabilities were generally regarded as unnecessary.
Recent terrorist actions have upset conventional thinking and created an impetus for protecting the cockpit of passenger aircraft from assault and penetration from would-be intruders in the passenger cabin. A higher level of protection may be attained by constructing the cockpit bulkhead and door from suitably strong, stiff, ballistic-resistant, and cut-resistant materials. Prior art aircraft interior panel materials are not adequately strong or stiff to protect the cockpit from a determined intruder, and possess limited or no resistance to penetration from ballistic objects. At the same time, prior art ballistic protection materials are too heavy, flexible, and/or expensive to be well suited for constructing walls and door in passenger aircraft. Accordingly, a lightweight ballistic resistant rigid structural panel is desired for applications such as passenger aircraft. The panel should be capable of being used like prior art structural panels for aircraft interiors, while offering a higher degree of resistance to both ballistic and high force/low speed attacks.
The present invention provides a rigid structural panel that resists penetration by ballistic objects such as bullets or shrapnel, or by sharp cutting objects such as knives or saws, while maintaining high rigidity and strength against high force/low speed attack. The structural panel is relatively lightweight, and may be handled and used similarly to prior-art structural panels in the manufacture of aircraft interior components such as doors or bulkheads. Among the advantages of the structural panel are a superior ability to resist high force/low speed attack through greater strength and rigidity, and an ability to maintain most of this strength and rigidity after a ballistic attack. The panel preferably has a thickness in the range of about 0.38 to 2.5 inches, and an area density (weight per unit area) in the range of about 1.8 to 2.5 pounds per square foot.
The structural panel is made up of a panel core including a plurality of sheets of flexible, high-tensile strength fabric interleaved with a plurality of sheets of a thermal-fusible film adhesive, and a sheet of cushioning material adhered to the plurality of sheets of flexible, high-tensile strength fabric. Fiber-reinforced face skins are adhered to exterior surfaces of the panel core for structural strength and rigidity.
The flexible, high-tensile strength fabric may be a woven or non-woven material, preferably made of a high-tensile fiber material such as aramid fiber, ultra-high molecular weight polyethylene fiber, or PBO fiber. Each of the sheets of flexible, high-tensile strength fabric should have a tensile breaking strength not less than about 100 pounds per inch of fabric width for every ounce per square yard of material       (                  lbf        /        in                    oz        /                  yd          2                      )    .
For example, a 16-ounce fabric should have a breaking strength not less than 1600 pounds. The panel core preferably has not less than 12 and not more than 33 sheets of high-strength fabric. As an assembly, the panel core is preferably not less than 0.25 inches thick, and not greater than 2 inches thick. The sheets of thermal-fusible film adhesive may be made of an ionic copolymer, epoxy, polyurethane, or other thermoplastic material. The thermal-fusible film adhesive is used in a quantity sufficient to consolidate the panel core and provide good structural strength and fatigue resistance, without sacrificing the ballistic resistance provided by the sheets of high-tensile strength fabric. The sheet of cushioning material may be selected from an aramid honeycomb material, an aluminum honeycomb material, a polyurethane foam material, or other cushioning materials. The sheet of cushioning material should have a density not less than 3 pounds per cubic foot and not greater than 8 pounds per cubic foot.
The face skins of the structural panel provide structural strength and rigidity to the panel. They also assist in flattening ballistic projectiles before the projectiles reach the panel core, thereby improving the ballistic resistance of the panel core. Suitable materials for the face skins include unidirectional glass fibers or fiberglass woven materials impregnated with a phenolic resin. The face skins are preferably not less than 0.01 inches thick, and not greater than 0.10 inches thick. They should have a tensile strength of 40,000 pounds per square inch, or higher.
In an embodiment of the invention, a hardened layer is adhered to an outer surface of the face skin facing the passenger cabin, to provide resistance to cutting by sharp objects. For example, a stainless steel mesh may be laminated to an outer surface of one of the face skins. In addition, to enhance resistance to attack by motorized cutting tools, one or more layers of gummy resin may be laminated to or within other layers in the panel core.
A more complete understanding of the lightweight ballistic resistant rigid structural panel will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.